Intestinal mucosal surfaces harbor a vast complex microbial ecosystem, the microbiome, which is intimately involved in host development and protection. One member of this ecosystem is Enterococcus faecalis (EF), a pathobiont that normally lives symbiotically with its host, but also can invade the host and cause systemic disease when intestinal homeostasis is disrupted. It is therefore an ideal model organism that can be used in a complex ecosystem to probe bacterial-host interactions that contribute to enterococcal niche formation in the GI tract. We hypothesize that EF establishes its niche in the GI tract through complex interactions between the bacterium and host. To investigate the host effect on the bacterium and the bacterium effect on the host we will use a novel mouse model for intestinal EF colonization. We will employ recombinase-based in vivo expression technology (RIVET) to investigate the impact of the host intestinal environment on EF (Aim 1). We will then determine the impact of EF colonization and bacteriocin production on host intestinal mucosal gene expression, using microarray technology. We anticipate that we will identify several microbial and host factors that are essential for EF's ability to establish stable colonization of he GI tract. With this knowledge we will begin to carry out specific analyses of both bacterial and host genes to determine their role in the process of EF niche formation in the GI tract. Ultimately, these findings will lead to a greater understanding of the complex bacterial-host interactions that allow persistent intestinal colonization by these multi-antibiotic resistant hospital-acquired bacteria, and offer insight into the interactions that result in enterococcal translocation and host systemic infection.